The present invention relates generally to data signal detectors, and more particularly to an improved method and apparatus for the detection of a data signal including repreated data words transmitted on a noisy communication channel, such as the radio channels found in some mobile and portable radiotelephone communication systems.
Prior communication systems have typically assigned separate communication channels to accommodate data signals and voice signals. However, in newer high-capacity mobile and portable radiotelephone communication systems (HCMTS), it is necessary to transmit data signals on voice communication channels in order to relay critical supervisory control information. For example, it is sometimes necessary to direct a mobile station of such an HCMTS system to switch from one voice channel to another. One such occasion necessitating voice channel switching results when a mobile or portable station moves from one cellular coverage area to another. Radiotelephone communication systems having these characteristics are described in a number of publications and patents, including, for example, U.S. Pat. Nos. 3,663,762, 3,819,872, 4,029,900 and 3,906,166. In these HCMTS systems, data is communicated over voice channels to an individual mobile or portable unit by briefly blanking the voice signals and sending a data signal during the blanking interval. Since the data signal is transmitted at a relatively high speed, for example, 10 KBPS, the subscriber hears only a momentary clock in the voice signal.
However, substantial difficulty is encountered in the mobile and portable stations in such HCMTS systems in recovering data signals transmitted on noisy voice channels. Therefore, it is desirable to utilize various techniques for enhancing reception of the data signal by the mobile and portable stations. For example, the information may be repeated a number of times within the data signal. Furthermore, each repeated information word may be preceded by a one-zero dotting pattern and a synchronization word, such as, a so-called Barker sequence, for increasing the probability of detection of the data signal by the mobile and portable stations. One such signalling scheme utilizing repeated information is described in the aforementioned U.S. Pat. No. 4,029,900. One format for the data signal described in this patent is illustrated herein in FIG. 2. The data signal of FIG. 2 includes eleven repeats of a data word, each preceded by a Barker word and a bit synchronization word.
According to a prior detection technique, the presence of the data signal may be determined by a tone detector that is tuned to the frequency of the dotting pattern in the bit synchronization signal. For example, if the bit frequency of the data signal is 10 KHz, the tone detector may be tuned to 5 KHz for detecting the one-zero dotting pattern of the bit synchronization signal. After detection of the bit synchronization signal, the following data words may be located by searching for the preceding Barker word. However, such a scheme is subject to falsing caused by interferring signals on the communication channel if the initial or following Barker words are not detected in sequence. For example, the Barker sequence may occur in the voice signal itself or may occur in the data words of the data signal. If the initial Barker word is missed, and a Barker sequence imbedded in a data word is detected, the entire data signal may not be detected, and as a result, may be totally ignored. Thus, the foregoing prior art detection technique may result in the loss of important supervisory control information when utilized to detect a data signal on a noisey voice channel of radio communication systems.